Environmental change and adaptation in Galápagos finches | Middle school biology | Khan Academy

This here is a picture of the ground finch of the Galapagos Islands, and one of its primary sources of food is seeds that it finds on the ground. If we go back to 1976, we can look at the distribution of beak depths, and these beak depths I would assume are given in millimeters. Finches are quite small birds, and you can see in 1976 you have a large number of finches that had a beak depth of 8.8 millimeters, but it was a distribution around that.

Now, after this data was collected, it turns out there was a drought, and there were fewer seeds. The smaller seeds, which were easy to eat for all of these finches, would have been consumed quickly, and all that would have been left were the larger seeds, the ones that you need a larger beak in order to crack and get at the good stuff.

So what do you think would have happened to the distribution of beak depths over the course of the next two years? Well, you might guess that the birds, the finches that have larger beak depths, are more likely to survive because they're more likely to be able to crack the larger seeds. The finches that are more likely to survive are also more likely to reproduce and pass their large beak trait to their offspring.

And that is indeed what scientists observed. As we go from 1976 to 1978, the distribution has shifted a pretty good bit to the right. Now, the most common beak depth is 9.8. So this is an example of an environmental change, a drought changing the food supply, because now there are fewer small seeds available. That changed a distribution in beak depths over just two years.

Now, environmental changes don't always cause adaptation. If it's too severe, if the drought was so strong that there were no seeds, you could have extinction; so the species disappears altogether. But here we have an example that in just two years, a species was actually unable to adapt a pretty good bit.

Now, no individual member of the species knew to somehow grow their beak, but as we saw, you always have a variation of beak depths. Year after year, the birds with the bigger beak depth were more likely to survive and reproduce. So the species as a whole adapted to the new environmental conditions.